Hello all,

Occasionally in C++ I find it useful to build an array which contains classes of multiple different types all using the same interface -- by constructing an array of pointers to some common base class, e.g.

class BaseClass {
    // blah, blah ...
};

class A : BaseClass {
    // ... blah ...
};

class C : BaseClass {
    // ... blah ...
};

int main()
{
    vector<BaseClass *> vec;
    vec.push_back(new A());
    vec.push_back(new C());
    // etc. etc.
}

(This code might be wrong; I'm just typing it to give the idea.  And in practice, I usually do not use 'new' statements but pass pointers to already-existing objects...:-)

Anyway, the point is that at the end of the day I have an array of different objects with a common interface.  What's the appropriate way to achieve the same effect in D?

Thanks & best wishes,

    -- Joe

On 24/04/10 20:06, Joseph Wakeling wrote:
> Hello all,
>
> Occasionally in C++ I find it useful to build an array which contains
> classes of multiple different types all using the same interface -- by
> constructing an array of pointers to some common base class, e.g.
>
> class BaseClass {
>      // blah, blah ...
> };
>
> class A : BaseClass {
>      // ... blah ...
> };
>
> class C : BaseClass {
>      // ... blah ...
> };
>
> int main()
> {
>      vector<BaseClass *>  vec;
>      vec.push_back(new A());
>      vec.push_back(new C());
>      // etc. etc.
> }
>
> (This code might be wrong; I'm just typing it to give the idea.  And in
> practice, I usually do not use 'new' statements but pass pointers to
> already-existing objects...:-)
>
> Anyway, the point is that at the end of the day I have an array of
> different objects with a common interface.  What's the appropriate way
> to achieve the same effect in D?
>
> Thanks&  best wishes,
>
>      -- Joe

This should do what you want:
----
class BaseClass {
    // blah, blah ...
}

class A : BaseClass {
    // ... blah ...
}

class C : BaseClass {
    // ... blah ...
}

int main()
{
    BaseClass[] vec;
    vec ~= new A;
    vec ~= new C;
    // etc. etc.
}
----

Robert Clipsham wrote:
> This should do what you want:

Thanks! :-)

Is it possible to do this with an interface instead of a base class? I'm not familiar with how the former work ...

Best wishes,

    -- Joe

On 04/25/2010 04:47 PM, Joseph Wakeling wrote:
> Robert Clipsham wrote:
>> This should do what you want:
>
> Thanks! :-)
>
> Is it possible to do this with an interface instead of a base class?
> I'm not familiar with how the former work ...
>
> Best wishes,
>
>      -- Joe

On 25/04/10 14:47, Joseph Wakeling wrote:
> Robert Clipsham wrote:
>> This should do what you want:
>
> Thanks! :-)
>
> Is it possible to do this with an interface instead of a base class?
> I'm not familiar with how the former work ...
>
> Best wishes,
>
>      -- Joe

Yes it is, providing the base doesn't implement any methods, eg:

----
interface I
{
  int foobar();
  // The following line will cause an error when uncommented, as
  // you cannot implement methods in an interface
  // void baz() {}
}

class C : I
{
  int foobar() { return 1; }
}

class D : I
{
  int foobar() { return 2; }
}

import std.stdio;

void main()
{
  I[] arr;
  arr ~= new C;
  arr ~= new D;
  foreach( el; arr )
    writefln( "%d", el.foobar() );
}
----
Prints:
1
2

You could also use an abstract class instead of an interface if you want to implement some of the methods.

Robert Clipsham wrote:

> interface I
> {
>   int foobar();
>   // The following line will cause an error when uncommented, as
>   // you cannot implement methods in an interface
>   // void baz() {}
> }

Just to be complete: interfaces can have static or final functions in D2:

    static void baz() {}
    final void baz_2() {}

Ali